近年来，我国大中型城市时常出现下水道爆炸事件。

In recent years, sewage gas explosion occur in many Chinese cities.

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近年来，小型和中型城市在全国各地取得了很大的改善。

In recent years, small and medium-sized cities throughout the country has made great improvement.

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近年来，小型和中型城市在全国各地取得了很大的改善。

In recent years, small and medium-sized cities throughout the country has made great improvement.

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